During development and after injury, neural cells migrate and elongate their axons towards proper target cells and organs in response to gradients of biomolecules, which guide axonal regeneration (chemotaxis) either by attachment to the cells or to the extracellular matrix (ECM), or by secretion into the extracellular fluid. In some cases, chemotactic soluble molecules are secreted by specific cells, and gradients are formed through diffusion and convection from the site of release. Cellular responses to such gradients can be influenced by the nature of the biomolecules, and physical characteristics of the ECM (which can include collagen, fibronectin, and laminin), such as matrix pore size and stiffness. In the developing peripheral nervous system (PNS), gradients of neurotrophic factors (NTF) such as nerve growth factor (NGF), neurotrophin 3 (NT-3), and brain-derived neurotrophic factor (BDNF), are established by distal target cells and direct axonal elongation and target recognition of motor neurons (VMN) from the ventral spinal cord, as well as sensory neurons in the dorsal root ganglia (DRG). In the adult PNS, the efferent branch of sensory neurons re-innervates skin and muscle targets spontaneously after injury, but afferent axons are unable to enter the hostile environment of the adult spinal cord, unless enticed by induced NGF expression. Moreover, pathfinding errors made by injured VMN and DGR neurons during regeneration can be dramatically reduced by the expression of the appropriate gene expression that re-establishes NTF gradients.
Unfortunately, the creation of chemical gradients such as NTF gradients and the use of chemical gradients in nerve repair remain extremely challenging. For example, some prior technologies fail to provide sustained release of desired molecular signals and/or lack ECM support. Other technologies lack the ability to provide non-transient and/or physiologically relevant chemical gradients normally present in vivo. Further, some previous methods for creating a chemical gradient are applicable to only short-term studies in vitro and/or present risks associated with the injection of a viral vector. Therefore, improved apparatuses and methods for providing a chemical gradient are desired.